The present invention relates generally to powder metallurgy. In particular, the invention relates to a method for manufacturing by powder metallurgy a composite material which is deformed in at least one dimension, which composite is formed from at least one and preferably two powder components, at least one of which is a body-centered cubic metal, and at least one of which may be contaminated with oxygen or an oxygen compound in its bulk or at its surface.
The manufacture of composite materials by powder metallurgy is known. Typically, two powder components, preferably two metal powders, are mixed, densified, extruded and drawn into wires by means of a deformation. In this manner, a composite material is produced in which the powder particles are drawn out into fibers, i.e., the composite fiber structure is deformed in two dimensions. If the composite material is deformed in only one dimension, the powder particles are drawn to form discontinuous ribbons.
In the manufacture of these composite materials, the two powder components are generally mixed, compacted, sintered and then subjected to the deformation process in which the powder particles are drawn out to form long fibers. However, particularly in the case of small powder particle sizes (less than or equal to 40 .mu.m) of the powder components, oxygen is dissolved interstitially and/or at the surface as the corresponding oxide through reaction with atmospheric oxygen and/or oxygen present during the processing of the powder. The interstitially dissolved oxygen increases the hardness of a body-centered cubic powder component as well as the critical temperature of the ductile to brittle transition. As a consequence, a polyphase structure made powder-metallurgically by a sintering process cannot be transformed into the desired fiber structure during cold deformation because the hardened powder components do not allow this deformation to the degree desired. Indeed, these hardened components are present, in effect, as undeformed particles or as multiply-torn fiber segments in the other nonhardened powder component or components. The mechanical properties of such a composite material are unfavorable. In particular, the breaking stress of such composites is low (W. D. Jones, "Fundamental Principles of Powder Metallurgy," Arnold, London 1960; Metallische Verbundwerkstoffe (Metallic Composite Materials), Festschrift of the firm G. Rau, Pforzheim 1977; Series of the Powder Metallurgy Joint Group of the Iron and Steel Institute and the Institute of Metals, London).
It has already been attempted to purify an oxygen-contaminated, body-centered cubic powder component, particularly niobium, by reduction with hydrogen at temperatures of about 1000.degree. C. However, the free binding enthalpies for interstitially dissolved oxygen and/or for the corresponding oxides, particularly in the case of transition metals, can amount to more than 100 (kcal/g-atom of oxygen) (at 25.degree. C.). As a result, reducing the metal with either hydrogen, carbon monoxide or the like at temperatures of up to 1000.degree. C. is often unsuccessful. (E. Fromm and E. Gebhardt, "Gase und Kohlenstoff in Metallen," Reine und angewandte Metallkunde in Einzeldarstellungen, Vol. 26, Springer Verlag Berlin 1976).